Ethernet bridges and tagged BPDUs

Archived from groups: comp.dcom.lans.ethernet (More info?)

Hi,

The scenario: I would like to create a transparent L2 connection
between two LANs via a third LAN, and run spanning tree between the
endpoints without interfering with the configuration of the
intermediate LAN switches. My idea is to VLAN tag the traffic between
the LANs at the endpoints, including the STP traffic. In that way I
hope that the intermediate switches will just forward the BPDUs used
for STP, and not look into the contents of these frames.

The question: How will an Ethernet switch react to incoming BPDUs that
are VLAN tagged? Or put another way: Will the swicthes in the
intermediate LAN

1) Change their STP configuration according to the BPDU
2) Discard the BPDU
3) Forward the BPDU with the VLAN tag preserved
4) or...?

I cannot seem to find anything in the IEEE standards about this.

If this doesn't work, are there alternatives?

BR,
Mike Absalon
3 answers Last reply
More about ethernet bridges tagged bpdus
  1. Archived from groups: comp.dcom.lans.ethernet (More info?)

    In article <2a8c329b.0407270304.319ab7d6@posting.google.com>,
    kajfas@hotmail.com (kajfas) writes:
    >Hi,
    >
    >The scenario: I would like to create a transparent L2 connection
    >between two LANs via a third LAN, and run spanning tree between the
    >endpoints without interfering with the configuration of the
    >intermediate LAN switches. My idea is to VLAN tag the traffic between
    >the LANs at the endpoints, including the STP traffic. In that way I
    >hope that the intermediate switches will just forward the BPDUs used
    >for STP, and not look into the contents of these frames.
    >
    >The question: How will an Ethernet switch react to incoming BPDUs that
    >are VLAN tagged? Or put another way: Will the swicthes in the
    >intermediate LAN
    >
    >1) Change their STP configuration according to the BPDU
    >2) Discard the BPDU
    >3) Forward the BPDU with the VLAN tag preserved
    >4) or...?
    >
    >I cannot seem to find anything in the IEEE standards about this.
    >
    >If this doesn't work, are there alternatives?

    Yes. Do not use STP. (As long as this is the only connection
    it is no use anyway).

    The KISS principle rules. Just build your networks in the most simple
    and stupid way possible. If anything breaks, let it break.
    Your average downtime will be significantly less than the outages
    introduced by the ill implemented features promising redundancy
    in todays equipment.

    --
    Manfred Kwiatkowski kwiatkowski@zrz.tu-berlin.de
  2. Archived from groups: comp.dcom.lans.ethernet (More info?)

    kajfas@hotmail.com (kajfas) wrote in message news:<2a8c329b.0407270304.319ab7d6@posting.google.com>...
    > Hi,
    >
    > The scenario: I would like to create a transparent L2 connection
    > between two LANs via a third LAN, and run spanning tree between the
    > endpoints without interfering with the configuration of the
    > intermediate LAN switches. My idea is to VLAN tag the traffic between
    > the LANs at the endpoints, including the STP traffic. In that way I
    > hope that the intermediate switches will just forward the BPDUs used
    > for STP, and not look into the contents of these frames.
    >
    > The question: How will an Ethernet switch react to incoming BPDUs that
    > are VLAN tagged? Or put another way: Will the swicthes in the
    > intermediate LAN
    >
    > 1) Change their STP configuration according to the BPDU
    > 2) Discard the BPDU
    > 3) Forward the BPDU with the VLAN tag preserved
    > 4) or...?
    >
    > I cannot seem to find anything in the IEEE standards about this.
    >
    > If this doesn't work, are there alternatives?

    It may be possible to do what you are trying to do depending
    on which switches you are using. Prior to multi-instance
    spanning tree in IEEE 802.1Q, spanning tree PDUs (or BPDUs)
    were never tagged. Since they are identified by bridges
    because of their special MAC address, a bridge would have
    been expected to:
    (a) never generate tagged BPDUs, and
    (b) discard any received BPDUs that were tagged.

    With newer switches that implement multi-instance spanning
    tree, you are allowed to configure "spanning tree domains"
    and it's possible you may be able to tunnel the BPDUs from
    one spanning tree domain across another. (I don't think
    the spec says anything about this, though.)

    I think you'd have to very carefully look into what your
    equipment does. There isn't an easy yes/no answer to this.

    Anoop
  3. Archived from groups: comp.dcom.lans.ethernet (More info?)

    "Anoop Ghanwani" <ghanwani@gmail.com> wrote in message
    news:67582204.0407282134.23f531b8@posting.google.com...
    > kajfas@hotmail.com (kajfas) wrote in message
    news:<2a8c329b.0407270304.319ab7d6@posting.google.com>...
    > > Hi,
    > >
    > > The scenario: I would like to create a transparent L2 connection
    > > between two LANs via a third LAN, and run spanning tree between the
    > > endpoints without interfering with the configuration of the
    > > intermediate LAN switches. My idea is to VLAN tag the traffic between
    > > the LANs at the endpoints, including the STP traffic. In that way I
    > > hope that the intermediate switches will just forward the BPDUs used
    > > for STP, and not look into the contents of these frames.
    > >
    > > The question: How will an Ethernet switch react to incoming BPDUs that
    > > are VLAN tagged? Or put another way: Will the swicthes in the
    > > intermediate LAN
    > >
    > > 1) Change their STP configuration according to the BPDU
    > > 2) Discard the BPDU
    > > 3) Forward the BPDU with the VLAN tag preserved
    > > 4) or...?
    > >
    > > I cannot seem to find anything in the IEEE standards about this.
    > >
    > > If this doesn't work, are there alternatives?
    >
    > It may be possible to do what you are trying to do depending
    > on which switches you are using. Prior to multi-instance
    > spanning tree in IEEE 802.1Q, spanning tree PDUs (or BPDUs)
    > were never tagged. Since they are identified by bridges
    > because of their special MAC address, a bridge would have
    > been expected to:
    > (a) never generate tagged BPDUs, and
    > (b) discard any received BPDUs that were tagged.
    >
    > With newer switches that implement multi-instance spanning
    > tree, you are allowed to configure "spanning tree domains"
    > and it's possible you may be able to tunnel the BPDUs from
    > one spanning tree domain across another. (I don't think
    > the spec says anything about this, though.)

    the implementation in various switches is specific to the manufacturer,
    varies by software release and config - and is not usually documented at the
    level needed to check what you want to do.

    you probably need to bench test this to find out what will happen.

    there is an alternative - several manufacturers support multiple 802.1q
    labels in a single packet - this setup is intended to let a carrier build a
    layer 2 shared network for multiple customers.

    Since one of the reasons for using label stacking (or Q in Q, or various
    other names) is to insulate the carrier from customer Spanning Trees - it
    should do what you want.
    >
    > I think you'd have to very carefully look into what your
    > equipment does. There isn't an easy yes/no answer to this.
    >
    > Anoop
    --
    Regards

    Stephen Hope - return address needs fewer xxs
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